Fuel cells are envisaged as an electric power supply system for future mass-produced motor vehicles as well as for a large number of applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. Hydrogen (H2) or molecular hydrogen is used as a fuel for the fuel cell. The molecular hydrogen is oxidized on an electrode of the cell and oxygen (O2) or molecular oxygen from the air is reduced on another electrode of the cell. The chemical reaction produces water. The great advantage of the fuel cell is that it averts emissions of atmospheric pollutant compounds at the place where electricity is generated.
One of the major difficulties in the development of such fuel cells lies in the synthesis and supply of dihydrogen (or molecular hydrogen). On earth, hydrogen does not exist in great quantities except in combination with oxygen (in the form of water), sulphur (in the form of hydrogen sulphide) and nitrogen (as ammonia) or carbon (fossil fuels such as natural gas or petroleum). The production of molecular hydrogen therefore requires either the consumption of fossil fuels or the availability of large quantities of low-cost energy in order to obtain this hydrogen from the decomposition of water, by thermal or electrochemical means.
One method for producing hydrogen from water consists of the use of the principle of electrolysis. To implement such methods, electrolyzers provided with proton-exchange membranes (PEMs) are known. In such an electrolyzer, an anode and a cathode are fixed on either side on the proton-exchange membrane and put into contact with water. A difference in potential is applied between the anode and the cathode. Thus, oxygen is produced at the anode by oxidation of water. The oxidation at the anode also gives rise to H+ ions that pass through the proton-exchange membrane up to the cathode, and electrons that are sent back to the cathode by the electrical supply unit. At the cathode, the H+ ions are reduced at the level of the cathode to generate molecular hydrogen.
Such an electrolysis device comes up against undesirable effects. The proton-exchange membrane is not perfectly impermeable to gas. A part of the gases produced at the anode and the cathode thus passes through the proton-exchange membrane by diffusion. This induces problems of purity of the gas produced but also induces problems of security. It is desired that the proportion of hydrogen in oxygen remain below 4%.
The permeability of the membranes to gas can be reduced by increasing the thickness of the proton-exchange membrane. This, however, causes an increase in the electrical resistance by making it more difficult for the H+ ions to pass through, and lowers the performance of the systems.
To limit the permeability of a proton-exchange membrane to gases, certain developments suggest a depositing of catalyst particles inside the proton-exchange membrane. The catalyst particles seek to recombine the molecular hydrogen passing through the membrane with molecular oxygen passing through the membrane. The quantities of molecular oxygen that reach the cathode and of molecular hydrogen that reach the anode are thus reduced.
However, the recombination reaction of the catalyst particles is exothermal and induces a loss of energy. Furthermore, such a solution is not optimized for industrial-scale applications since a part of the molecular hydrogen generated at the cathode is nevertheless lost inside the proton-exchange membrane. Furthermore, the permeability of the proton-exchange membrane to molecular hydrogen is greater than its permeability to molecular oxygen. Consequently, a part of the molecular hydrogen nevertheless reaches the anode since the quantity of molecular oxygen is insufficient in the catalyst particles disposed in the membrane.
In order to achieve low cost, efficient, and durable fuel cells, the fabrication process of the known MEAs needs to be improved.